LEACHING AND DISSOLVING
i.e., the ratio of water to solids. Second the strength phosphoric acid de the amount of acid to be produced and amount of solids residues.
The first condition will control the number of thickeners required, any washing efficiency up to 100 per cent can be obtained by this n
To CVAPO«ATO« on STORAGE
T- TONS OF WATCR
FIG. 8.—Counter-current leaching of sodium hydroxide.
practically it may not be feasible where the amount of solution carried frr~ to thickener by the solids is very great. The methods of computing the such a system is shown in the following problem. Conditions Assumed: From Fig. 8.
1. Ten tons NaOH produced per 24 hr.
2. Overflow from thickener X to have approximate specific gravity 1.1 (190 Ib. NaOH per ton).
3. Yield of NaOH or "causticity" of finished product 92 per cent.
4. Time of agitation 2 hr.
5. Lime mud discharged from each thickener with approximately 1 part water to 1 part solids by weight.
Let X, Y and Z equal tons of dissolved NaOH per ton of water in the respective thickeners.
Equating total tons NaOH out of and into each thickener, we have:
1. 105.2X + 17X = 122.27 + 10 tons (NaOH)
2. 122.27 + 177 = 17 X + 122.2Z
3. 122.2Z + 17X = 177 Solving:
X = 7.327 = 0.0948 tons = 189.6 Ib. NaOH per ton. 7 = S.20Z = 0.0129 tons = 25.8 Ib. NaOH per ton. Z = 0.00158 = 3.16 Ib. NaOH per ton. Conclusions:
The 105.2 tons going to storage will contain at 189.6 Ib. NaOH per ton or 9.974 tons NaOH.
This amount is 99.7 per cent of total 10 tons produced.
The Dorr classifier and the Dorr thickener it will be noted are designed to handle two different types of material. The classifier handles quick-settling sandy material which does not require long-washing periods while the thickener is best adapted to handling slow-settling slimes which require longer periods of detention in the washes.
Centrifugals.—A method sometimes used for removing the solution from solids is washing them in centrifugals. This method is of course expensive both in labor and power and is only resorted to in special cases. This method is used in washing the film of syrup from the crystals in sugar refining. The sugar and syrup are first spun for about 1 min. to remove the greater part of the syrup and then an extremely small wash applied by automatic washers. (It is of interest